Add Warning: NAT Gateway not covered by Free Tier

So I used this example without knowing that NAT Gateways are not covered under AWS's free tier, which in about two weeks racked up $30 in charges.

Please add a note to the README for this project warning developers that they can be charged a significant amount of money by deploying this code. I can only imagine what would have happened if I had deployed this and left it up all month.